1. Field of the Invention
The field of the invention generally relates to low noise figure radiofrequency (RF) devices such as tower mounted amplifiers (TMA's) used in wireless applications. More specifically, the field of the invention relates to the specific design features that are used to produce both a low noise figure and rejection in compact devices. These designs have applications in Tower Mounted Amplifiers (“TMAs”) or Mast-Head Amplifiers (“MHAs”), Tower Mounted Boosters (“TMBs”) or any other application where low noise figure, rejection of interfering signals and compact size are requirements such as, for example, remote RF applications, and repeater applications.
2. Description of the Related Art
As mobile usage increases, wireless service providers are increasingly faced with the challenge of optimizing and/or expanding their wireless networks to provide better service for their customers while also minimizing their network capital expenditures. TMAs (or MHAs) and TMBs are currently being used extensively in wireless networks to improve the range of cellular base stations. Generally, a TMA or MHA consists of a filter and low noise amplifier (“LNA”) which is mounted at or near the top of a base station tower. TMAs and MHAs improve signal quality by boosting the uplink (Rx) signal of a mobile system immediately after the antenna. TMAs and MHAs compensate for the loss in signal that occurs in the coaxial cable run from the antenna to the base transceiver station (“BTS”). The goal of TMAs and MHAs is to amplify the in-band signal close to the antenna so as to provide the lowest possible noise contribution to the overall receiver system. The noise figure of the TMA should be as low as possible, so as to minimize the noise from the TMA itself. For example, a difference of 1 dB in the noise figure results in a difference in coverage of .about.20%. Thus, even small changes in the noise figure as measured in dBs (which are measured on a logarithmic scale) can significantly affect the overall performance characteristics of the device.
TMAs and MHAs can result in increased coverage area for a given base station. This allows mobile subscribers to place more calls, place longer calls, increase data throughput, as well as reduce the number of dropped calls. This also reduces the overall number of base stations required to cover a specific area, hence, minimizing overall capital and operating expenditures. Capital expenditures for wireless towers/base stations can be very expensive. For example, it is estimated that each tower/base station can cost between about $500,000 and $750,000. In addition, each base station requires ongoing site lease expenses, backhaul (such as T1 lease), maintenance, totaling .about.$50,000 per year. Because if of this, TMAs and MHAs have the ability to significantly reduce overall capital and operating expenditures on wireless infrastructure because a lower number of towers/base stations may be used to provide the same amount of coverage for a particular area.
TMAs or MHAs have become increasingly used as wireless carriers move to higher frequencies (i.e., greater than about 1.5 GHz) because RF propagation losses are much higher and service range is much shorter at these frequencies (as compared to .about.850 MHz—the initial deployment frequency of cellular in the United States) and .about.900 MHz or 450 MHz (initial deployment frequency in Europe). TMAs or MHAs are typically overlaid on top of existing base station infrastructure in order to avoid the high cost to site and construct additional base station towers. Current TMAs or MHAs rely on metal resonator/air-filled, cavity-based filters which can have low loss but poor filtering characteristics or good filtering characteristics and high loss. It is important, however, to reduce out-of-band signals as much as possible because signals passing through the filters will be amplified and passed to the BTS. This is particularly important because the presence of out-of-band interfering signals will produce additional noise in the system because of spurious signals generated within the non-linear components such as the LNA and mixers, or saturation of the components resulting in significantly reduced performance from the system.
The problem is that in order to mount the LNA as close as possible to the antenna, the filter in the TMA or MHA must necessarily be small because of the limited space or “real estate” at the top of the tower. In current metal resonator/air cavity-based filters, this necessitates poor filtering performance. While high performance metal resonator/cavity filters are available, their large size and increased loss precludes them from being used in close-to-the antenna applications (e.g., in TMA or MHA systems).
Thus, there is a need for a RF device that provides excellent out-of-band signal rejection with low pass band loss, yet is small enough to mount close to the antenna. Preferably, the RF device can be incorporated into TMAs or MHAs which can be overlaid on existing tower infrastructure for use in a wide variety of frequencies including 2 GHz (or higher) applications.
In addition, there is a growing need for better filtering in newer (3G) air interfaces such as CDMA and OFDM. This need for better filtering comes from the fact that on CDMA and OFDM wireless networks, any interference has a significant impact on the receiver performance, unlike earlier protocols such as analog, TDMA or GSM. Furthermore, data services are becoming increasingly important to wireless carriers. Unfortunately, data is much less forgiving than voice with respect to errors. The power amplifier design is much more complex and is limited by the out of the band emissions at maximum power. This can, however, be reduced with good filtering. Thus, newer technologies being implemented in wireless networks are driving the need for good filtering on both the transmit and the receive side of the network.